Liquid water seems simple, but there’s a lot of chemistry going on in it.

It is common knowledge that, in pure water, under ordinary conditions, 1 in every 10 million H2O molecules is dissociated into the acid ion H⁺ and the base ion OH^–. However, what preference, if any, these self-ions of water have to sit at the air water interface has been the subject of lengthy and heated debate. The question is consequential in a wide range of contexts, including on the surface of droplets in the atmosphere and at the surfaces of biomolecules.  The Herzfeld group has now bridged the gap between experiment and theory by using a model that efficiently balances three subtle features of water molecules (polarizability, H+ sharing, and H+ transfer) that control the ambient behavior of the liquid. The model predicts that OH– prefers the air-water interface while H+ avoids it, consistent with observations of the response of air bubbles in water to an applied electric field.

Bai C, Herzfeld J. Surface Propensities of the Self-Ions of Water. ACS Central Science. 2016.